RFI suppression circuit and method

Abstract

Improvement of radar sign-to-noise ratio and detection sensitivity in radar systems is achieved by methods employing the subtraction of the unwanted radio frequency interference, RFI, or "clone" signals thereof, from the total received signal. The Clone signals are appropriately adjusted in phase and amplitude, and are obtained from an auxilliary broad beam antenna or from a delayed sample from the system's principal antenna. When multiple RFI signals at different frequencies are present, the entire receive band is subdivided into a plurality of frequency sub-ranges.

What is claimed is: 
     
       1. In a system for transmitting and receiving pulsed RF signals, comprising a transmitter for radiating pulsed RF signals, a directional antenna, and a broad band receiver coupled to said directional antenna for receiving and processing pulsed RF signals of a given frequency in the possible presence of narrow band RFI signals created by a plurality of extraneous narrow band RFI sources, said narrow band RFI being distinct in frequency from one another and from said given frequency, whereby said receiver incidentally detects said RFI signals in addition to detecting pulsed wide band RF signals of said given frequency, the improvement for reducing the effect of said RFI signals, comprising: separator means for parsing received signals amongst a plurality of separate and distinct frequency channels in dependence on the frequency of said signals and separately outputting each said channel, whereby a plurality of received RFI signals of different frequencies is produced at the respective channel outputs associated with such frequencies and wherein a received pulsed RF signal is divided into constituent spectral components and said constituent spectral components are individually output from respective channels associated with respective constituent spectral components;   counter signal generating means for producing a counter signal for each said channel; each said counter signal corresponding to at least a respective RFI signal in a respective one of said plurality of channels, each respective counter signal being of essentially the same amplitude as and phase with said associated respective RFI signal;   first combiner means, responsive to said separator means and to said counter signal generating means, for subtractively combining signals from each of said plurality of channels with the respective ones of said plurality of counter signals from said counter signal generating means to produce a corresponding plurality of difference signals, wherein each said RFI is effectively minimized and wherein the constituent components of a received pulsed RF signal are presented; and   second combiner means, coupled to said first combiner means, for summing said plurality of difference signals to provide a composite output signal reproducing said pulsed RF signal.   
     
     
       2. The invention as defined in claim 1, wherein said counter signal generating means comprises: a plurality of delay means, said plurality corresponding in number to said plurality of channels, and each of said delay means being associated with a corresponding one of said channels, each said delay means being connected to receive the output of the respective channel and output a delayed signal to a corresponding input of said first combiner means, whereby a delayed version of the RFI in the associated channel is supplied to said first combiner means.   
     
     
       3. The invention as defined in claim 1 wherein said counter signal generating means comprises: Omnidirectional broad beam antenna means for receiving RFI signals propagating from virtually any direction to thereby duplicate RFI signals received by said receiver means;   second receiver means for receiving said RFI signals from said omnidirectional broad beam antenna means;   second separator means for parsing received signals amongst a plurality of separate and distinct frequency channels and separately outputting each said channel, whereby a plurality of received RFI signals falling within different frequency sub-ranges is produced at the respective channel outputs associated with such frequencies and wherein any received pulsed RF signal is divided into a plurality of constituent spectral components and parsed amongst said plurality of channels;   a plurality of delay means and amplitude adjustment means, with each of said delay means being serially connected with a corresponding one of said amplitude adjusting means, each of said delay means having an input connected to a respective output of a respective one of said channels defined by said second separator means; and each said amplitude adjustment means having a corresponding output to provide a plurality of outputs of said separator means.   
     
     
       4. The invention as defined in claim 1, wherein said separator means comprises: a plurality of tuned bandpass filters, each of said tuned bandpass filters having a defined bandwidth and a defined frequency range for selectively passing signals, said defined frequency ranges for said tuned bandpass filters being collectively equal to said predetermined frequency range of said receiver.   
     
     
       5. The invention as defined in claim 1, wherein said transmitter comprises a radar transmitter for transmitting pulsed RF signals, said receiver comprises a radar receiver for receiving radar echoes, and said pulsed RF signal comprises a radar echo. 
     
     
       6. The invention as defined in claim 2, wherein each said delay means includes phase shifting means. 
     
     
       7. The invention as defined in claim 6, wherein each said delay means is adjustable. 
     
     
       8. The invention as defined in claim 1 wherein said first combiner means comprises a plurality of combining circuit means, one associated with each said channel of said separator means, for subtractively combining signals from an associated channel with a respective one of said plurality of counter signals from said counter signal generating means, to produce a difference signal, each of said combining circuit means further comprising: differential amplifier means. 
     
     
       9. The invention as defined in claim 1, further comprising: a plurality of gain controlled amplifier means, each of said gain controlled amplifier means being connected in circuit between a respective output of said first combiner means and an input of said second combiner means, and each said gain controlled amplifier means for regulating the average power level of signals inputted to said second combiner means.   
     
     
       10. The invention as defined in claim 7 further comprising: a plurality of automatic adjustment means for adjusting the amount of delay of each of said plurality of delay means of said counter signal generating means to minimize the output of said first combiner means during the presence of RFI signals at said input to said first combiner means and said counter signal generating means during the absence of a pulse RF signal.   
     
     
       11. The invention as defined in claim 7 further comprising: automatic adjustment means for adjusting the amount of delay of said counter signal generating means to minimize the output of said first combiner means during the presence of RFI signals at said input to said first combiner means and said counter signal generating means during the absence of a pulse RF signal, comprising: sample and hold means;   switch means;   compare and logic circuit means for analyzing signal levels and for controlling said time delay means of said counter signal generating means;     said switch means being connected between the output of said first combiner means and said compare and logic circuit means;   said sample and hold means for closing said switch means during the absence of a pulsed RF signal at said receiver means to provide a sample of the difference signal from said first combiner means to said compare and logic circuit means;   said compare and logic circuit means determining from consecutive samples of said difference signal any change in the level of said difference signal and, responsive to determination of an increase in level, for adjusting said time delay of said counter signal generating means to produce a decrease in the level of said difference signal, whereby the amount of RFI that can pass through said first combiner means is minimized.   
     
     
       12. The invention as defined in claim 7 further comprising: a plurality of automatic adjustment means for adjusting the amount of delay of each of said plurality of delay means of said counter signal generating means to minimize the output of said first combiner means during the presence of RFI signals at said input to said first combiner means and said counter signal generating means during the absence of a pulse RF signal, each of said plurality of automatic adjusting means, comprising: sample and hold means;   switch means;   compare and logic circuit means for analyzing signal levels and for controlling said time delay means of said counter signal generating means;     said switch means being connected between the output of said first combiner means and said compare and logic circuit means;   said sample and hold means for closing said switch means during the absence of a pulsed RF signal at said receiver means to provide a sample of the difference signal from said first combiner means to said compare and logic circuit means;   said compare and logic circuit means determining from consecutive samples of said difference signal any change in the level of said difference signal and, responsive to determination of an increase in level, for adjusting said time delay of said counter signal generating means to produce a decrease in the level of said difference signal, whereby the amount of RFI that can pass through said first combiner means is minimized.   
     
     
       13. The invention as defined in claim 3, wherein each said delay means is adjustable. 
     
     
       14. The invention as defined in claim 13, wherein each said delay means comprises an adjustable phase adjustment circuit and and adjustable gain control circuit in series circuit. 
     
     
       15. The invention as defined in claim 14 further comprising: a plurality of automatic adjustment means for adjusting the amount of delay of each of said plurality of delay means of said counter signal generating means to minimize the output of said first combiner means during the presence of RFI signals at said input to said first combiner means during the absence of a pulsed RF signal.   
     
     
       16. The invention as defined in claim 13, further comprising: a plurality of automatic adjustment means for adjusting the amount of delay of each of said plurality of delay means of said counter signal generating means to minimize the output of said first combiner means during the presence of RFI signals at said input to said first combiner means during the absence of a pulse RF signal, each of said plurality of automatic adjusting means, comprising: sample and hold means;   switch means;   compare and logic circuit means for analyzing signal levels and for controlling said time delay means of said counter signal generating means;     said switch means being connected between the output of said first combiner means and said compare and logic circuit means;   said sample and hold means for closing said switch means during the absence of a pulsed RF signal at said receiver means to provide a sample of the difference signal from said first combiner means to said compare and logic circuit means;   said compare and logic circuit means determining from consecutive samples of said difference signal any change in the level of said difference signal and, responsive to determination of an increase in level, for adjusting said time delay of said counter signal generating means and for adjusting said gain to produce a decrease in the level of said difference signal, whereby the amount of RFI that can pass through said first combiner means is minimized; and   chopper means for alternately coupling the output of said compare and logic circuit means to said phase adjustment circuit and said gain adjustment circuit.   
     
     
       17. The invention as defined in claim 1, wherein said separator means comprises: an analog to digital converter for converting signals into digital form;   a Fourier transformer responsive to said converter for digitally processing said signal and parsing said digitally processed signal into separate digital channels, each said channel defining a separate frequency range; and   wherein each of said first combiner means, counter signal generating means and said second combiner means comprises, respectively, a digital first combiner means, a digital counter signal generating means and a digital second combiner means.   
     
     
       18. The invention as defined in claim 17, further comprising: feedback control loop for adjusting said digital counter signal generating means.   
     
     
       19. The method of preventing a plurality of spurious RF sources of different frequencies from producing interference with the detection and processing of desired radar echoes in a wide band pulse radar receiving system capable of receiving such different frequencies, comprising the steps of: separating received signals collectively representing spurious interference signals and the desired radar echo into a plurality of frequency sub-ranges, whereby each spurious signal falls into a respective one of said sub-ranges to produce a separate plurality of spurious signals of individual amplitude and phase together with a component of any desired radar echo that is simultaneously received;   generating a counter signal for each separate spurious signal, each said counter signal being equal in amplitude and phase with the amplitude and phase determined from the spurious signal within each respective subrange;   subtracting each counter signal from the corresponding spurious signal plus said desired echo component in each said subrange to produce a corresponding difference signal that is essentially a component of the desired radar echo;   combining all said difference signals to produce a combined signal in which any interference signals have been minimized, whereby the combined signal represents essentially the desired radar echo.   
     
     
       20. The method as defined in claim 19, wherein the step of generating a counter signal for each spurious signal includes the step of delaying a portion of the spurious signal that is received during the absence of said desired radar echo. 
     
     
       21. The method as defined in claim 19, wherein the step of generating a counter signal for each spurious signal includes the steps of: simultaneously separately receiving said spurious interference signals to provide a second source of said spurious interference signals;   separating received signals from said second source into a plurality of frequency sub-ranges corresponding to the sub-ranges of said first source, whereby each spurious signal falls into a respective one of said sub-ranges to produce a separate plurality of spurious signals of individual amplitude and phase;   adjusting the amplitude and phase of each spurious signal from said second source in each subrange to match the amplitude and phase of spurious signals to generate the counter signal for each subrange.   
     
     
       22. The method as defined in claim 21, wherein the step of adjusting the amplitude and phase of each spurious signal from said second source in each subrange to match the amplitude and phase of spurious signals to generate the counter signal for each subrange, includes the steps of: determining whether a radar echo is being received; and   continuously adjusting the amplitude and phase only during the periods in which a radar echo is not being received.   
     
     
       23. A radar system for transmitting and receiving a train of pulsed RF signals, whereby a reflecting surface on which said pulsed RF signals is incident reflects echoes thereof to said radar system, said radar system comprising a transmitter, directional antenna, receiver, said receiver receiving from said directional antenna and detecting a total signal occuring within a predefined pass band comprising an echo of a transmitted RF pulse signal and any RFI present within said pass band, and a signal processor for receiving and processing RF echoes during the presence of RFI, said signal processor including circuit means for reducing the effect of said RFI in the received detected signal, comprising: means for determining the amplitude of the detected RFI existing at the time of echo reception; and   means for subtracting the amplitude of said detected RFI from the amplitude of the total detected signal to produce a difference signal representative of the desired echo, whereby said RFI is substantially eliminated.   
     
     
       24. The invention as defined in claim 23, wherein said means for deterimining the amplitude of the detected RFI at the time of echo reception comprises: means for sampling and averaging the RFI occuring during a first short time interval prior to reception of a desired echo signal and a second short time interval subsequent to reception of said desired echo signal; and   delay means for delaying the sampled signals for a third predetermined time interval equal to the time between the completion of said sampling to the next reception of RFI that follows shortly the next received echo.   
     
     
       25. The invention as defined in claim 23, wherein said means for subtracting the amplitude of said detected RFI from the amplitude of the total detected signal to produce a difference signal representative of the desired echo, comprises: differential amplifier means. 
     
     
       26. The invention as defined in claim 23, wherein said means for determining the amplitude of the detected RFI existing at the time of echo reception, comprises: broad beam antenna means for receiving RFI separately from any RFI received with said echo at said directional antenna means; and   means for adjusting the level of RFI received at said broad beam antenna to be equal to that received at said directional antenna.